The purpose of this study was to examine the role of steroid hormones in the urinary concentrating mechanism by examining separately, with the isolated tubule technique, the effect of steroids on the hydraulic water permeability (Lp) of the collecting tubule and on solute transport by the thick ascending limb of Henle's loop. An additional objective, considering the instability of the response to vasopressin in rabbit collecting tubules, was to examine the mouse as a possible alternative animal for the perfused tubule studies. Once appropriate dissection techniques were developed, the function of mouse collecting tubules was examined. The Lp response to vasopressin was five times that seen in the rabbit and more importantly, the response was stable for several hours. However, the response of collecting tubules from adrenalectomized and sham-operated animals were equally responsive to vasopressin. Thick ascending limbs of Henle's loop were also obtained from normal mice and, prior to testing for their response to steroid hormones, the response to vasopressin was examined. Vasopressin produced a reversible increase in electrical potential but had no effect on Lp. This data provides the first evidence that vasopressin has a functional effect on this segment of the nephron. The data to date thus provides evidence for a new site of action of vasopressin where solute rather than water, is primarily affected and demonstrate the potential for studies in the mouse, another mammal which can be routinely perfused, but the first which can also be used for micropuncture studies.